Tuning indicator for frequency modulation receivers



Jan. 30, 1951 E. H. SCHOENFELD 9,

TUNING INDICATOR FOR FREQUENCY MODULATION RECEIVERS Filed March 51. 1948m PULSE- gum/m INVENTOR Y EARL E-I. SEHEENEELD ATTORNEY F'atented Jan.30, 1951 TUNING INDICATOR FOR FREQUENCY MODULATION RECEIVERS Earl H.Schoenfeld, Mamaroneck, N. Y., assignor to Radio Corporation of America,a corporation of Delaware Application March 31, 1948, Serial No. 18,121

12 Claims.

This invention relates to tuning indicator circuits and particularly toa method of and circuit for visually indicating the proper tuning of anangle-modulated carrier-wave receiver.

An amplitude-modulated carrier-wave receiver (AM receiver) is usuallytuned to the desired station by adjusting its tuning control knob untilthe volume of the reproduced music or speech has a maximum value.However, a frequency-modulated carrier-wave receiver (FM receiver)cannot be tuned in the same manner because the volume of the reproducedsound has a maximum value not only when the receiver is correctly tunedbut also when the receiver input circuit is adjusted to resonateslightly above or below the center frequency of the FM wave. When the FMreceiver is tuned above or below the center frequency of the FM wave,the demodulated signal will be distorted. Various circuits haveaccordingly been proposed for indicating the proper tuning of an FMreceiver. Usually tuning of the receiver is indicated visually by meansof a so-called tuning eye which is an electronic indicator tube having afluorescent target on which a shadow is produced which varies with thetuning indication signal.

In one proposed circuit the tuning eye is connected to the grid circuitof the limiter stage which is conventionally provided aheadof thefrequency discriminator of an FM receiver. However, when the amplitudeof the FM wave is comparatively high, the grid voltage of the limiterdoes not vary over a certain frequency range including the centerfrequency of the FM wave so that tuning cannot be accomplished with sucha circuit. Furthermore, certain types of frequency discriminators suchas a ratio detector do not require a limiter stage, and in that casethis tuning indicator circuit cannot be used. A conventional ratiodetector has been described in the book FM Simplified, by Milton S.Kiver, published by D. Van Nostrand Co., Inc., New York, on page 140.

Another tuning indicator circuit has been suggested where the audiooutput signal derived from the frequency discriminator is utilized forobtaining a tuning indication. In this circuit, the instantaneous audiosignal is integrated to derive a direct current voltage which is thenimpressed directly on a tuning eye. The tuning eye must be biassed sothat the shadow seen on its fluorescent screen closes when the receiveris correctly tuned. When the receiver is not correctly tuned, the shadowseen on the tuning eye either opens or overlaps. However, this tuningindicator circuit has the disadvantage that the correct bias voltage forthe tuning eye depends on the amplitude of the received FM wave whichmay vary Within wide limits unless a limiter stage is provided ahead ofthe frequency discriminator. A circuit of this type, accordingly, cannotbe used with a ratio detector which does not require the use of alimiter stage. i

It is a principal object of the present invention, therefore, to providea novel method of and circuit for visually indicating the proper tuningof an FM receiver which does not require a limiter stage for derivingthe indicating signal.

A further object of the invention is to derive a signal representativeof the proper tuning of an FM receiver from the balanced discriminatorof the receiver.

Another object of the invention is to provide, in an FM receiverincluding a ratio detector, a method of and circuit for developing asignal indicative of the proper tuning of the receiver from themodulation signal derived from the ratio detector, the signal beingarranged to vanish when the receiver is properly tuned.

An FM receiver may comprise a balanced frequency discriminator such, forexample, as a ratio detector, having an output circuit for developing asignal representative of the modulation component of an angle-modulatedcarrier wave. The expression angle-modulated carrier wave is meant toinclude a frequencymodulated carrier wave (FM wave) or a phasemodulatedcarrier Wave or a hybrid wave possessing characteristics common to bothphase and frequency-modulated waves.

A tuning indicator circuit which illustrates the principles of thepresent invention may include, for example, an integrating circuitcoupled to the output circuit of the frequency discriminator forderiving a direct current voltage which is representative of the averageor mean value of the modulation signal. A chopper, which may either be amechanical chopper or an electronic device, is coupled to theintegrating circuit for periodically interrupting the direct currentvoltage. The direct current voltage is thus transformed into acorresponding alternating current voltage which has an amplitude that isindependent from the polarity of the direct current voltage but dependsonly upon its absOlute ni ud This, alternating current voltage is thenrectified by a rectifier. Finally, an indicator such as a tuning eye iscoupled to the rectifier for indicating the magnitude of the rectifiedvoltage. When the FM receiver is .properly tuned, this rectified voltagewill vanish.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organiza-..

tio and method of operation, as well as additional objects andadvantages thereof, will best be understood from the followingdescription when read in connection with the accompanying drawing, inwhich:

Fig. 1 is a circuit diagram, partly in block form, of an FM receiverincluding a ratio detector and provided with a tuning indicator circuitembodying the present invention;

Fig. 2 is a graph illustrating the instantaneous audio output voltagederived from the ratio detector of Fig. l in response to frequencyvariations of the received FM wave;

Figs. 3, 4 and 5 are graphs illustrating the interrupted voltage derivedfrom the tuning indicator circuit of Fig. 1 when the receiver ismistuned;

Fig. 6 is a schematic front view of a tuning eye employed with thetuning indicator circuit of the invention to illustrate the shadowvisible on its target;

Figs. '7 and 8 are schematic views of difierent mechanical chopperswhich may be utilized in the circuit of Fig. 1;

Fig. 9 is a circuit diagram of an electronic chopper in accordance withthe invention controlled by pulses or saw-tooth waves; and

Fig. 1G is a circuit diagram of an alternative electronic chopper inaccordance with the invention controlled by a sinusoidal wave.

Referring now to 1 there is illustrated an FM receiver including a ratiodetector and provided with a tuning indicator circuit in accordance withthe present invention. An anglemodulated carrier wave may be interceptedby antenna l and amplified by one or more radiofrequency amplifierstages, converted to an inter mediate-frequency wave and furtheramplified by one or more intermediate-frequency amplifier stagescontained within box 2. The radio-frequency amplifier and converterstages may be tuned to a desired wave by varying a reactance such ascondenser 3 shown schematically by means of tuning control knob 4. Thelast intermediate-frequency amplifier contained within box 2 has a tunedoutput circuit 5 including coil 6 resonated by condenser l. Tuned outputcircuit 5 is resonant at the center frequency of theintermediate-frequency wave.

A ratio detector indicated Within dotted lines If! is coupled to tunedoutput circuit 5. Ratio detector It comprises a frequency discriminatornetwork which includes tuned circuit i i consisting of coil [2 resonatedby condenser it. The

midpoint of coil i2 is connected to the high alt-e13 nating potentialterminal of coil 5 through coupling condenser I l. Furthermore, coils 8and I2 are inductively coupled. Two diodes l5 and i6 are coupled to thefrequency discriminator net- Work. Thus, the cathode of diode i 5 isconnected to the upper terminal of coil 52, while the lowerterminal ofthe coil is connected to the anode of diode l6. Load condensers ll andwin-series are connected to the output electrodes of diodes l5 and I6,that is, to the anode of diode l5 and to' the cathode of diode I6.Resistor'Zll shunts load condensers ll, l8 and is accordingly alsoconnected to the anode of diode l5 and to the cathode of diode 16.Preferably, the midpoint of resistor 26 is grounded. Bypass condenser 2ishunts re sistor 2i). Condenser 2| has a low impedance forintermediate-frequency and audio-frequency currents while loadcondensers ll, l8 have a low impedance for intermediate-frequencycurrents but a high impedance for audio-frequency currents.

The junction point A of load condensers l1, I8 is connected to themidpoint of coil 12 through coil 22 forming a return path for theintermediate-frequency currents passing through the two diodes. Thedemodulated audio signal is derived from junction point A and impressedthrough coupling condenser 23 on audio amplifier 24 and reproduced byloud speaker 25.

In accordance with the present invention a tuning indicator signal isdeveloped from the discriminator-output signal. To this end theinstantaneous output signal derived from junction point A is firstintegrated to develop the average or direct current component of thediscriminatoroutput signal. This is accomplished by series resistor 28and shunt condenser 39, one terminal of which is grounded. Seriesresistor 28 and shunt condenser 36 represent a conventional integratingor filter circuit, and it is to'be understood that any otherconventional integrating circuit may be substituted for resistor 28 andcondenser as. The thus developed direct current voltage is impressedthrough lead 29 on chopper 3! indicated by a box. Various mechanical orelectric choppers which may be substituted for box 3 I will be explainedhereinafter in connection with Figs. Ito 10. Chopper 3! willperiodically interrupt the direct current voltage, and the interruptedoutput signal 32 derived from chopper 3| shown in Fig. l is impressedthrough lead 39 and coupling condenser 33 upon diode rectifier 34 havingits cathode grounded as shown. The rectified signal is developed acrossload resistor 35 which shunts diode 34.

The rectified signal derived from diode 34 is impressed through seriesresistor 36 upon control grid 3'! of tuning eye indicator 33.. Bypasscondenser Ml is provided between control grid 37 and ground forbypassing the alternating current component of the rectified signal.Tuning eye 38 which may be of the 6E5 type has a triod'e sectionincluding cathode ll, control grid 31 and anode A2. Cathode 4! isgrounded while anode 4 2 is connected to a suitable positive voltagesource +5 through anode resistor $3. The sec.- ond section of tuning eye38 includes cathode 44 connected to cathode ll and fluorescent target 55on which a shadow of anode G2 is thrown which has a variable width.

The angle-modulated carrier wave intercepted by antenna 1 is amplifiedand converted to an intermediate-frequency wave in a conventional mannerby the components included within box 2. The amplifiedintermediate-frequency wave is impressed upon the frequencydiscriminator network. When the frequency of' the wave equals the centerfrequency, equal voltages are impressed upon diodes IE and I6, and anoutput signal of zero amplitude is derived from junction point A of loadcondensers l! and i8. However, when the frequency of the wave deviatesfrom its center frequency, the voltages impressed upon diodes l5-and I6are no longer equal, .and the voltage of junction point A becomespositive or negative depending upon whether the frequency of' the waveis above or belowits center'frequency.

This "is illustrated in Fig. 2 where curve 50,

component that is positive.

which is substantially astraight line, indicates the instantaneous-audiooutput voltage derived from point A in response to frequency-variationsofthe input wave from the center frequency fc. It will be noted that theoutput voltage is zero when the frequency of the wave equals its centerfrequency. The voltage developed across resistor 20 and bypass condenser2| is maintained substantially constant for time variations which areshort compared to one cycle of the audio signal thereby to rejectundesired amplitude modulations of the FM wave.

However, the mean or direct current component of the audio outputvoltage derived from point A is only zero when the radio-frequencyamplifier and converter stages are properly tuned by tuning control knob4 tothe center frequency of the received wave. Let it be assumed thatthe receiver is tuned to a frequency above the center frequency of thereceived wave. In that case, the audio output signal derived fromjunction point A may have an average or direct current This average ordirect current voltage component is derived from integrating circuit 28,30 and impressed upon chopper 3| where it is periodically interrupted.The output wave derived from chopper 3| under these conditions .isillustrated in Fig. 3 and consists of pulses 5| which are positive withrespect to ground. On the other hand, if the receiver is mistuned to afrequency below the center frequency f0, the instantaneous audio signalderived from point A will include a negative direct current voltagecomponent which is filtered by integrating circuit 28, 30 andinterrupted by chopper 3|. The resulting pulses 52 are illustrated inFig. 4 and have a negative polarity with respect to ground.

Pulses 5| and 52 are now impressed upon diode rectifier 34 throughcoupling condenser 33 and, accordingly, pulses 5| and 52 are centered onthe alternating current axis 53 or 54 respectively. The position of axis53 or 54 only depends on the duration of each pulse 5| or 52 compared tothe time between consecutive pulses. The resulting pulses which arederived from coupling condenser 33 and impressed upon diode rectifier 34are illustrated at 55 in Fig. 5. Pulses 55 have an amplitude which isindependent of the polarity of the direct current voltage and whichdepends only upon the absolute magnitude of the direct current voltage.When pulses 55 are impressed upon diode rectifier 34, a negative voltageis developed across resistor 35 which is impressed through resistor 36on control grid 31 of tuning eye 38. Consequently, as illustrated inFig. 6 fluorescent target 45 of tuning eye 38 has a narrow shadow 56 aslong as the receiver is not properly tuned.

As tuning control knob 4 is adjusted to tune the receiver to the correctfrequency the amplitude of pulses 55 decreases and consequently. avoltage which is less negative is impressed upon control grid 3! oftuning eye 38. The tuning eye accordingly opens and the shadow increasesin size or width as indicated in Fig. 6 in dotted lines 51.

The FM receiver of Fig. 1 is therefore tuned by adjusting tuning controlknob 4. until the shadow 5T seen on the target 45 of tuning eye 38mistuned to. the same extent above or :below resonance. Thus, the signalimpressed upon con-i. trol grid 31 of the tuning eye varies withtheifrequency to which the receiver is tuned'inthe manner indicated by lines58 of Fig. 2. This,

signal decreases to zero when the receiver is properly tuned. Unless thedirect current voltage derived from point A is interrupted in the mannerdescribed, tuning eye 38 will not indicate the,

proper tuning of the receiver because the indicating signal would simplychange its polarity;

when the receiver is tuned through the current frequency, and the tuningeye is not responsive to a signal of such character. a

Referring now to Fig. 7 there is illustratedfa mechanical chopper whichmay be substituted for box 3| in Fig. 1. The chopper of Fig. '7comprises a flat metallic disc 60 having inserts 6| of insulat-- ingmaterial. Disc 60 is rotated by a suitable motor 62 through theintermediary of a drive shaft schematically indicated at 63. The averageor direct current voltage is impressed through.

lead 23 on brush 64 riding on the circumference of disc 50. Brush B5slides over the metallicportion of disc 50 and is connected to outputlead 38 which, in turn, is connected to coupling condenser 33. Lead 39is grounded through resistor 59 to provide a discharge path forcondenser 33. When disc 68 rotates, the voltage impressed upon couplingcondenser 33 is periodically interrupted and condenser 33 isperiodically discharged.

Referring now to Fig. 8 there is illustrated an is impressed uponcoupling condenser 33 throughlead 39.

Referring now to Fig. 9 there is illustrated an electronic chopper whichmay be substituted for box 3| of Fig. l. Triode 10 has its anodeconnected to the junction point between series resistor 28 and couplingcondenser 33. The cathode of triode I0 is grounded. Triode 1| has itscathode connected to the junction point between series resistor 28 andcoupling condenser 33 while its anode is grounded. The control grid oftriode 10 is connected to pulse generator 12 which may develop pulses 13of positive polarity. Output pulses 14 of negative polarity are alsodeveloped by pulse generator 12 and impressed on the control grid oftriode i.

It will now readily be seen that when the receiver of Fig. 1 is tunedabove the center frequency of the FM wave a positive voltage isdeveloped and impressed upon the anode'of triode 10 and the cathode oftriode 1|. Triode 10 will accordingly conduct whenever its control gridis rendered suificiently positive with respect to its cathode by a pulse13. Triode U will not conduct space current.

center frequency of the desired wave, a negative voltage is impressedupon the anode of triode 10- and the cathode of triode H. In this case,triode 10 will not conduct. Triode II will normally condenser 33 causesthe same tuning indication to duct space current but will beperiodically rendered nonconducting whenever its control grid is drivensufficiently negative by a pulse 14.

The circuit of Fig. 9 may be used with advans.

tage in theaudio channel of a television receiver. In that case, pulsesl3 and 74 may be derived from the synchronizing pulses of the compositevideo signal. Alternatively, a positive saw-tooth wave 75 may beimpressed on the control grid of triode 10 and a negative saw-toothwavemay be impressed on the control grid of triode 1 I. Alternatively,the same symmetrical saw-tooth wave may be simultaneously impressed onthe control grids of both triodes l and H. Saw-tooth waves I5 and 16 maybe derived from the deflection generators of a television receiver.

Instead of impressing a pulse 13 of positive polarity on the controlgrid of triode ll] and a pulse 74 of negative polarity on the controlgrid of triode H, it is feasible to impress the same pulse having apositive and a negative portion above and below zero voltage on bothcontrol grids of triodes and H. In any case, the signal impressed uponcoupling condenser 33 is periodically grounded or interrupted at a ratecorresponding to the repetition rate of pulses 13, M or of saw-toothwaves 15, 16.

Referring now to Fig. 10 there is illustrated an alternative electronicchopper which may be substituted for box 3! in Fig. 1. The electronicchopper comprises triodes 'lll and ll. Triode 70 has its anode connectedto the junction point between Series resistor 28 and coupling condenser23, while triode H has its cathode connected to the same junction point,the cathode of triode 10 and the anode of triode H being g ounded. Asinusoidal wave is impressed through grid resistor 80 upon the controlgrids of triodes 10 and H. Grid resistor 80 preferably has a largeresistance value. The electronic chopper of Fig. 10 operates in the samemanner as that of Fig. 9 except that triodes l0 and ll are renderedconductive by a sinusoidal wave.

If the receiver is tuned to a frequency above the center frequency ofthe FM wave, a positive direct current voltage is impressed upon theanode of triode Ill and the cathode of triode 1i. Triode H3 will berendered conductive whenever the wave impressed upon its control gridexceeds the cut-oil voltage. During that period of time couplingcondenser 33 is discharged to ground potential. During the remainder ofthe cycle of the sinusoidal wave triode 70 remains nonconducting. TriodeH does not conduct current under these conditions. On the other hand,when the receiver is tuned to a frequency below the center frequency ofthe FM wave to be received, a negative voltage is impressed upon theanode of triode I0 and the cathode of triode H. Triode 10 will notconduct space current under these conditions. Triode H, on the otherhand, will be rendered conducting whenever its control grid is drivenbeyond the cut-off voltage with respect to the cathode, that is, duringthe positive portion of the sine wave. During that period of time,coupling condenser 33 is discharged and brought to ground potential.

There has thus been described a circuit for developing a signalindicative of the proper tuning vention has been shown connected to aratio detector. It is to be understood, however, that the tuningindicator circuit of the present invention may be used in connectionwith an balanced frequency discriminator provided its instantaneousaudio output voltage varies with variations of the frequency in themanner illustrated by curve 50 of Fig. 2. All that is necessary is tofeed the audio output signal into filter circuit 28, 30 of the tuningindicator circuit of the present invention.

What is claimed is:

1; A tuning indicator circuit for a balanced frequency discriminatorcomprising an output circuit coupled to said discriminator fordeveloping a signal representative of the modulation component of anangle-modulated carrier Wave, an integrating circuit coupled to saidoutput circuit for deriving a direct current voltage representative ofthe average value of said signal, a chopper coupled to said integratingcircuit for periodically interrupting said direct current voltage, arectifier coupled to said chopper for rectifying said interrupted directcurrent voltage, and an indicator coupled to said rectifier forindicating the magnitude of said rectified voltage.

2. A tuning indicator circuit for a balanced frequency discriminatorcomprising an output circuit coupled to said discriminator fordeveloping a modulation signal representative of the modulationcomponent of an angle-modulated carrier wave, a filter circuit coupledto said output circuit for deriving a direct current voltagerepresentative of the average value of said signal, a chopper coupled tosaid filter circuit for periodically interrupting said direct currentvoltage, a rectifier coupled to said chopper for rectifying saidinterrupted direct current voltage, a condenser coupled between saidchopper and said rectifier and a tuning eye indicator coupled to saidrectifier for visually indicating the magnitude cf said rectifiedvoltage.

3. In a frequency-modulated carrier wave receiver, a balanced frequencydiscriminator for deriving a modulation signal from said Wave, saiddiscriminator comprising a frequency discriminator network for derivingfrom said wave a pail of voltages whose relative magnitudes aredependent upon the frequenoy modulation of said wave, two rectifiers,said rectifiers being coupled to said network to apply each of saidvoltages to one of said rectifiers, a rectifier output circuit coupledto said rectifiers for deriving a modulation signal having an averagevalue which is zero when said receiver is properly tuned to said wave; atuning indicator circuit comprising an integrating circuit coupled tosaid rectifier output circuit for deriving a direct current voltagerepresentative of the average value of said modulation signal, a choppercoupled to said integrating circuit for periodically interrupting saiddirect current voltage, a further rectifier coupled to said chopper forrectifying said interrupted direct current voltage, and an indicatorcoupled to said further rectifier for indicating the magnitude of saidrectified voltage.

4. In a frequency-modulated carrier-wave receiver, a balanced frequencydiscriminator for deriving a modulation signal from said wave, saiddiscriminator comprising a frequency discriminator network for derivingfrom said'wave a pair of voltages whose relative magnitudes aredependent upon the frequency modulation of said wave, two rectifiers,said rectifiers being coupled to said network to apply each of saidvoltages to one of said rectifiers, a rectifier output circuit coupledto said rectifiers for deriving a modulation signal having an averagevalue which is zero when said receiver is properly tuned to said wave; atuning indicator circuit comprising a filter circuit coupled to saidrectifier output circuit for deriving a direct current voltagerepresentative of the average value of said modulation signal, a choppercoupled to said filter circuit for periodically interrupting said directcurrent voltage,

a further rectifier coupled to said chopper for.

rectifying said interrupted direct current voltage, a condenserconnected in series between said chopper and said further rectifier, anda tuning eye indicator coupled to said further rectifier for indicatingthe magnitude of said rectified voltage.

5. In a frequency-modulated carrier wave receiver, a balanced frequencydiscriminator for deriving a modulation signal from said wave, saiddiscriminator comprising a frequency discriminator network for derivingfrom said wave a pair of voltages whose relative magnitudes aredependent upon the frequency modulation of said wave, two rectifiers,said rectifiers being coupled to said network to apply each of saidvoltages to one of said rectifiers, a rectifier output circuit coupledto said rectifiers for deriving a modulation signal having an averagevalue which is zero when said receiver is correctly tuned to said wave;a tuning indicator circuit comprising an integrating circuit coupled tosaid rectifier output circuit for deriving a direct current voltagerepresentative of the average value of said modulation signal, two spacedischarge tubes connected between said integrating circuit and a pointof fixed potential, a source of control waves coupled to said tubes forperiodically rendering one of said tubes conducting to bring said directcurrent voltage, to said fixed potential, a further rectifier coupled tosaid chopper for rectifying the interrupted direct current voltage, andan indicator coupled to said further rectifier for indicating themagnitude of said rectified voltage.

6. In a frequency-modulated carrier wave receiver, a balanced frequencydiscriminator for deriving a modulation signal from said wave, saiddiscriminator comprising a frequency discriminator network for derivingfrom said wave a pair of voltages whose relative magnitudes aredependent upon the frequency modulation of said wave, two rectifiers,said rectifiers being coupled to said network to apply each of saidvoltages to one of said rectifiers, a rectifier output circuit coupledto said rectifiers for deriving a modulation signal having an averagevalue which is zero when said receiver is properly tuned to said wave atuning indicator circuit comprising an integrating circuit coupled tosaid rectifier output circuit for deriving a direct current voltagerepresentative of the average value of said modulation signal, two spacedischarge tubes connected be tween said integrating circuit and a pointof fixed potential, asource of control waves coupled to said tubes, saidtubes being connected in such a manner that one of said tubes isrendered periodically conducting depending on the polarity of saiddirect current voltage to bring said direct current voltage to saidpotential, a further rectifier coupled to said chopper for rectifyingsaid interrupted direct current voltage, a condenser connected in seriesbetween said chopper and said further rectifier, and an electronicindicator coupled to said further rectifier for indicating the magnitudeof said rectified voltage.

7. In an angle-modulated carrier-wave receiver, a ratio detectorcomprising a frequency discriminator network, two rectifiers coupled tosaid network, a rectifier output circuit coupled to said rectifiers andincluding a pair of load condensers connected in series to the outputelectrodes of said rectifiers, a resistor connected to said rectifieroutput electrodes and in parallel with said condensers to form a directcurrent path with said rectifiers, a modulation signal output circuitconnected to the junction point of said lead condensers; a tuningindicator circuit for indicating the average voltage between saidjunction point and the midpoint of said resistor, said average voltagebeing zero when said receiver is properly tuned to the wave to bereceived, said tuning indicator circuit comprising an integratingcircuit coupled to said junction point for deriving a direct currentvoltage representative of the average value of said signal, a choppercoupled to said integrating circuit for periodically interrupting saiddirect current voltagefa further rectifier coupled to said chopper forrectifying the interrupted direct current voltage, a condenser connectedin series between said chopper and said further rectifier, and anindicator coupled to said further rectifier for indicating the magnitudeof said rectified voltage.

8. In an angle-modulated carrier-Wave receiver, a ratio detectorcomprising a frequency discriminator network, two rectifiers coupled tosaid network, a rectifier output circuit coupled to said rectifiers andincluding a pair of load condensers connected in series to the outputelectrodes of said rectifiers, a resistor connected to said rectifieroutput electrodes and in parallel with said condensers to form a directcurrent path with said rectifiers, a modulation signal output circuitconnected to the junction point of said load condensers; a tuningindicator circuit for indicating the average voltage between saidjunction point and the midpoint of said resistor, said average voltagebeing zero when said receiver is properly tuned to the wave to bereceived, said tuning indicator circuit comprising an integratingcircuit coupled to said junction point for deriving a direct currentvotage representative of the average value of said signal, a chopperconnected between said integrating circuit and said midpoint forperiodically bringing said direct current voltage to the potential ofsaid midpoint, a further rectifier coupled to said chopper forrectifying the interrupted direct current voltage, and an indicatorcoupled to said further rectifier for indicating the magnitude of saidrectified voltage.

9. In an angle-modulated carrier-wave receiver, a ratio detectorcomprising a frequency discriminator network, two rectifiers coupled tosaid network, a rectifier output circuit coupled to said rectifiers andincluding a pair of load condensers connected in series to the outputelectrodes of said rectifiers, a resistor connected to said rectifieroutput electrodes and in parallel with said condensers to form a directcurrent path with said rectifiers, a modulation signal output circuitconnected to the junction point of said load condensers; a tuningindicator cir- .cuitfor indicating the average voltage between saidjunction point and the midpoint of said resistor, said average voltagebeing zero when said receiver is properly tuned to the wave to bereceived, said tuning indicator circuit comprising an integratingcircuit connected between said junction point and said midpoint forderiving a direct current voltage representative of the average value ofsaid signal, a chopper connected between said integrating circuit andsaid midpoint for periodically bringing said direct current voltage tothe potential of said midpoint, a further rectifier coupled to saidchopper for rectifying the interrupted direct current voltage, acondenser connected in series between said chopper and said furtherrectifier, and a tuning eye indicator coupled to said further rectifierfor indicating the magnitude of said rectified voltage.

10. The method of indicating tuning of an angle-modulated carrier-wavereceiver which comprises demodulating said Wave to derive a modulationsignal, filtering said modulation signal to derive a mean direct currentvoltage, transforming said mean voltage into a corresponding alternatingcurrent voltage, rectifying said alternating current voltage, andutilizing said rectified voltage for tuning indication.-

11. The method of indicating tuning of an angle-modulated carrier-wavereceiver which comprises demodulating said wave to derive a 30modulation signal, integrating said modulation signal to derive anaverage direct current voltage, periodically interrupting said averagevoltage to develop a corresponding alternating current voltage,rectifying said'alternating current voltage, and utilizing saidrectified voltage for tuning indication.

12. The method of indicating tuning of an angle-modulated carrier-wavereceiver which comprises demodulating said Wave to derive a modulationsignal, filtering said modulation signal to derive a mean direct currentvoltage, developing an alternating current voltage representative of theamplitude of said mean voltage regardless of its polarity, rectifyingsaid alternating current voltage, and obtaining a tuning indication inaccordance with the magnitude of said rectified voltage.

EARL H. SCHOENFELD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,175,690 Happe Oct. 10, 19392,280,607 Roberts Apr. 21, 1942 2,286,413 Herold et al. June 16, 1942

